The graph of 3x - 4y = 12 on the coodinate-plane is a line.
When x = 0 on the line 3x - 4y = 12, we see that the y-coordinate of the point on the line must satisfy 3(0) - 4y = 12. Or -4y = 12. Or y = -3.
So when x = 0 on the line, we ave y = -3. Therefore, (0, -3) is on the line.
Similarly, when y = 0 on the line, the point must satisfy 3x - 4(0) = 12.
So 3x = 12. Or x = 4. So (4, 0) is on the line. (Note that the x-coordinate comes first.)
Repeat that for y = -6. You're just pluggin -6 in for y and solving for x.